Solid-state imaging device

ABSTRACT

In each photosensitive cell, a photodiode  101 , a transfer gate  102 , a floating diffusion layer section  103 , an amplifier transistor  104 , and a reset transistor  105  are formed in one active region surrounded by a device isolation region. The floating diffusion layer section  103  included in one photosensitive cell is connected not to the amplifier transistor  104  included in that cell but to the gate of the amplifier transistor  104  included in another photosensitive cell adjacent to the one photosensitive cell in the column direction. A polysilicon wire  111  connects the transfer gates  102  arranged in the same row, and a polysilicon wire  112  connects the reset transistors  105  arranged in the same row. For connection in the row direction, only polysilicon wires are used.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.13/185,199, filed on Jul. 18, 2011 which is a Divisional of U.S.application Ser. No. 12/720,549, filed on Mar. 9, 2010, now U.S. Pat.No. 8,004,026, which is a Divisional of U.S. application Ser. No.11/396,485, filed on Apr. 4, 2006, now U.S. Pat. No. 7,696,543, which isa Divisional of U.S. application Ser. No. 10/753,954, filed on Jan. 9,2004, claiming priority of Japanese Patent Application No. 2003-062282,filed on Mar. 7, 2003, the entire contents of each of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a MOS-type solid-state imaging deviceto be used for various types of equipment, such as home video cameras,digital still cameras, or cameras incorporated in cellular phones.

2. Description of the Background Art

With reference to FIGS. 9 and 10, a conventional sensor and a scheme ofdriving the conventional sensor are described below. FIG. 9 is a circuitdiagram of the conventional sensor. The sensor illustrated in FIG. 9includes photosensitive cells (each surrounded by a dotted line)arranged in a 2×2 matrix. Each photosensitive cell includes a photodiode51, a transfer gate 52, a floating diffusion layer section 53, anamplifier transistor 54, a reset transistor 55, and an addresstransistor 56. Also, each photosensitive cell corresponds to one pixelfor forming an image. Note that, for the purpose of simplifyingdescriptions, it is assumed herein that the photosensitive cells arearranged in a 2×2 matrix. In practice, however, the photosensitive cellsare arranged in a matrix with several tens to thousands of rows andcolumns. Also, in FIG. 9, the components included in the samephotosensitive cell are provided with the same suffix (a through d) foridentification.

The scheme of driving the sensor illustrated in FIG. 9 is describedbelow. In order to extract signals from the photosensitive cells on thefirst row, the address registers 56 a and 56 b included in the firstphotosensitive cells on the first row are first controlled by a verticalshift register 61 to be in an ON state. Next, the reset transistors 55 aand 55 b are controlled also by the vertical shift register 61 to be inan ON state. With this, the floating diffusion layer sections 53 a and53 b are reset. At this time, the amplifier transistor 54 a and a loadtransistor 63 p form a source follower circuit, producing an output on avertical signal line 62 p. Similarly, the amplifier transistor 54 b anda load transistor 63 q form a source follower circuit, producing anoutput on a vertical signal line 62 q. Here, voltages appearing on thevertical signal lines 62 p and 62 q are noise voltages irrespectively ofsignal electric charges stored in the photodiodes 51 a and 51 b. Next,the transfer gates 52 a and 52 b are controlled by the vertical shiftregister 61 to be in an ON state. Thus, the signal electric chargesstored in the photodiodes 51 a and 51 b are transferred to the floatingdiffusion layer sections 53 a and 53 b, causing signal voltagescorresponding to the signal electric charges stored in the photodiodes51 a and 51 b to appear on the vertical signal lines 62 p and 62 q.

Clamp capacitors 64 p and 64 q, clamp transistors 65 p and 65 q,sample/hold transistors 66 p and 66 q, and sample/hold capacitors 67 pand 67 q form a noise suppression circuit. This noise suppressioncircuit finds a difference between a pixel output with a signal electriccharge being applied to the floating diffusion layer section 53 (thatis, a signal output) and a pixel output with a signal electric chargenot being applied thereto (that is, a noise output). Noise occurring atthe sensor illustrated in FIG. 9 mainly includes noise caused byvariations in threshold voltage at the amplifier transistor 54 and kTCnoise, which is thermal noise occurring at the reset transistor 55. Whennoise outputs appear on the vertical signal lines 62 p and 62 q, theclamp transistors 65 p and 65 q and the sample/hold transistors 66 p and66 q are controlled by control terminals 74 and 75 to be in an ON state.Also at this time, the sample/hold capacitors 67 p and 67 q are appliedwith a noise-suppressed clamp voltage through a clamp-voltage supplyterminal 73. After a predetermined time has elapsed, the clamptransistors 65 p and 65 q are controlled by the control terminal 74 tobe in an OFF state.

Next, voltages each being equal to a sum of the noise-suppressed signalvoltage and the noise voltage appear on the vertical signal lines 62 pand 62 q. With this, the noise voltages on the vertical signal lines 62p and 62 q are each changed to the sum of the signal voltage and thenoise voltage, and each amount of change corresponds to eachnoise-suppressed signal voltage. Accordingly, the voltages at thesample/hold side of the clamp capacitors 64 p and 64 q are also changedby the amount corresponding to the noise-suppressed signal voltage. Inpractice, the voltages applied to the sample/hold capacitors 67 p and 67q are each changed from a noise-suppressed clamp voltage by the amountcorresponding to a voltage obtained by dividing the amount of change inthe signal voltage on a corresponding one of the vertical signal lines62 p and 62 q by a corresponding one of the clamp capacitors and acorresponding one of the sample/hold capacitors. Therefore, the voltageapplied to each of the sample/hold capacitors 67 p and 67 q is the sumof the noise-suppressed clamp voltage and the divided signal voltage,and noise has been suppressed. After the sample/hold transistors 66 pand 66 q are controlled to be in an OFF state, horizontal transistors 68p and 68 q are sequentially and selectively controlled by a horizontalshift register 69 to be in an ON state. With this, signals correspondingto the signal electric charges stored in the photodiodes 51 a and 51 bare sequentially output from an output terminal 70.

Next, in order to extract signals from the photosensitive cells on thesecond row, an operation similar to that performed on those on the firstrow is performed on those on the second row. With this, signalscorresponding to the signal electric charges stored in the photodiodes51 c and 51 d are output from the output terminal 70.

The above-described operation is illustrated in a timing chart as inFIG. 10. In FIG. 10, a period during which signals stored in one row ofthe photodiodes 51 are eventually output from the output terminal iscalled a horizontal effective period, while a period during whichsignals are output from the photodiode 51 to the vertical signal line 62for suppression of noise included in the output signals is called ahorizontal blanking period. Furthermore, the horizontal blanking periodand the horizontal effective period are collectively called a horizontalperiod. The horizontal period is a time period actually required forreading signals of one row. A time period required for reading signalsfrom the entire sensor is called one frame period. As illustrated inFIG. 10, the amount of signal electric charge stored in the photodiode51 depends on a time interval of a transfer pulse being applied to thetransfer gate 52. Also, the time interval of the transfer pulse isconstant during one frame period. Therefore, the sensitivity of thephotodiode 51 is constant.

In the sensor illustrated in FIG. 9, each photosensitive cell is formedby four transistors (the transfer gate 52, the amplifier transistor 54,the reset transistor 55, and the address transistor 56). By contrast, insome sensors recently devised for achieving size reduction, eachphotosensitive cell is formed by three transistors. Such a newly devisedsensor has a structure in which the address transistor 56 is omittedfrom the sensor illustrated in FIG. 9 and a power source is shared amongthe photosensitive cells. In order to allow signals to be read from thissensor, a pulse-type source voltage is required to be supplied to eachphotosensitive cell.

The scheme of driving the sensor as illustrated in FIG. 9 is disclosedin, for example, Japanese Patent Laid-Open Publication No. 9-247537(1997-247537). As for a sensor in which photosensitive cells are eachformed by three transistors, no documents describing a specific layoutof these photosensitive cells are known.

In the above sensor as well as a semiconductor integrated circuit, thecircuit layout is a key to determine the size of the circuit, as well asthe circuit configuration and design rules. In general, as the circuitsize is smaller, yields of the circuit are improved more, therebyreducing the cost of the circuit. Therefore, laying out a circuitaccording to predetermined design rules is an important technical taskin designing the semiconductor integrated circuit. However, as for asensor in which photosensitive cells are each formed by threetransistors, no specific layout of these photosensitive cells has beenclearly known to public.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to clarify a newcircuit configuration of a sensor in which photosensitive cells are eachformed by three transistors, and to provide a sensor of small circuitsize.

The present invention has the following features to attain the objectmentioned above.

A solid-state imaging device of the present invention outputs anelectrical signal in accordance with an intensity of a received opticalsignal. The solid-state imaging device includes a photosensitive regionhaving photosensitive cells two-dimensionally arranged in row and columndirections on a semiconductor substrate, a power supply line, a verticaldriver circuit, a plurality of vertical signal lines, a plurality ofload registers, a noise suppression circuit, a plurality of horizontaltransistors, and a horizontal driver circuit. Each of the photosensitivecells includes a photodiode for storing a signal electric chargeobtained by performing optical-electrical conversion on received light,a transfer transistor for transferring the signal electric charge storedin the photodiode, a floating diffusion layer section for temporarilystoring the transferred signal electric charge, an amplifier transistorfor amplifying the signal electric charge stored in the floatingdiffusion layer section, and a reset transistor for resetting the signalelectric charge stored in the floating diffusion layer section. Thepower supply line is commonly connected to drains of the amplifiertransistors. The vertical driver circuit drives the transfer transistorsarranged in a same row and the reset transistors arranged in anothersame row. The vertical signal lines are commonly connected to theamplifier transistors arranged in a same column. The load transistorsare respectively connected to the vertical signal lines. The noisesuppression circuit suppresses noise of signals output to the verticalsignal lines. The horizontal transistors are arranged in the rowdirection and are supplied with outputs from the noise suppressioncircuit. The horizontal driver circuit sequentially and selectivelycauses the horizontal transistors to be operated so as to cause theoutputs to be sequentially produced from the noise suppression circuit.In each photosensitive cell, the photodiode, the transfer transistor,the floating diffusion layer section, the amplifier transistor, and thereset transistor are formed in one active region surrounded by a deviceinsulation region, and the floating diffusion layer section included ina given one of the photosensitive cells is connected to a gate of anamplifier transistor included in a photosensitive cell adjacent to thegiven photosensitive cell in the column direction.

With this, it is possible to attain a circuit configuration of thephotosensitive cells for achieving a suitable layout without impairingthe function of the solid-state imaging device. This makes it possibleto reduce the size of the laid-out photosensitive cells and, in turn,reduce the circuit size of the entire sensor.

In this solid-state imaging device, a plurality of first horizontalsignal lines commonly connected to gate electrodes of the transfertransistors arranged in the same row and a plurality of secondhorizontal signal lines commonly connected to gate electrodes of thereset transistors arranged in the same row can be made of a samematerial. Also, the floating diffusion layer section in a given one ofthe photosensitive cells can be located between the first horizontalsignal line connected to the gate electrode of the transfer transistorincluded in the same photosensitive cell and the second horizontalsignal line connected to a gate electrode of the reset transistorincluded in the given photosensitive cell.

With the first horizontal signal line and the gate electrode of thetransfer transistor, and the second horizontal signal line and the gateelectrode of the reset transistor being made of the same material, nocontact holes are required for routing these signal lines. Therefore, itis possible to reduce the size of the laid-out photosensitive cells,thereby reducing the entire size of the entire sensor.

Also, a first contact hole can be provided on the floating diffusionlayer section so as to connect the floating diffusion layer section anda gate of the amplifier transistor, a second contact hole can beprovided on a common drain shared by the amplifier transistor and thereset transistor so as to connect the common drain to the power supplyline, a third contact hole can be provided on a source of the amplifiertransistor so as to connect the source to a corresponding one of thevertical signal lines, and a fourth contact hole can be provided on awiring area where the gate of the amplifier transistor is located so asto connect the floating diffusion layer section and the gate of theamplifier transistor. Further, the first contact hole, the secondcontact hole, the third contact hole, and the fourth contact hole may bealigned approximately n a straight line. Still further, the firstthrough third contact holes may be aligned approximately in a straightline.

With the plurality of contact holes being aligned approximately in astraight line in the layout of the photosensitive cells, it is possibleto reduce an area required for layout of these contact holes. Therefore,it is possible to reduce the size of the laid-out photosensitive cells,thereby reducing the entire size of the entire sensor.

Still further, a signal line connecting the floating diffusion layersection and the gate of the amplifier transistor, the power supply line,and the vertical signal lines are formed in a same metal wire layer.

With the signal line which connects the floating diffusion layer sectionand the gate of the amplifier transistor, the power supply line, and thevertical signal lines being formed by the same metal wire layer, nocontact holes are required for routing these signal lines. Therefore, itis possible to reduce the size of the laid-out photosensitive cells,thereby reducing the entire size of the entire sensor.

Still further, the power supply line may include a plurality of verticalpower supply lines commonly connected to drains of the amplifiertransistors arranged on the same column. Also, a signal line connectinga given one of the floating diffusion layer sections and a gate of acorresponding one of the amplifier transistors together may be locatedbetween one of the vertical signal lines which is connected to theamplifier transistor of one of the photosensitive cells which includesthe given floating diffusion layer section and one of the vertical powersupply lines which is connected to a drain of the correspondingamplifier transistor.

With this, the layout pattern of the photosensitive cells can be madesimple and regular. Therefore, it is possible to reduce the size of thelaid-out photosensitive cells, thereby reducing the entire size of theentire sensor.

Still further, all of the transistors included in each of thephotosensitive cells are N-channel MOS transistors.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an exemplary sensor which is referred tofor describing the present invention;

FIGS. 2A and 2B are detailed illustrations of a noise suppressioncircuit of a sensor according to an embodiment of the present invention;

FIG. 3 is a timing chart showing a scheme of driving the sensoraccording to the embodiment of the present invention;

FIG. 4 is a diagram illustrating a layout of the exemplary sensor whichis referred to for describing the present invention;

FIG. 5 is a diagram illustrating another layout of the exemplary sensorwhich is referred to for describing the present invention;

FIG. 6 is a circuit diagram of the sensor according to the embodiment ofthe present invention;

FIG. 7 is a diagram illustrating a layout of the sensor according to theembodiment of the present invention;

FIG. 8 is a diagram illustrating another layout of the sensor accordingto the embodiment of the present invention;

FIG. 9 is a circuit diagram of a conventional sensor; and

FIG. 10 is a timing chart showing a scheme of driving the conventionalsensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Prior to descriptions of a sensor according to an embodiment of thepresent invention, an exemplary sensor in which photosensitive cells areeach formed by three transistors is described. The exemplary sensorillustrated in FIG. 1 includes photosensitive cells (each surrounded bya dotted line) arranged in an m×n matrix, a power supply line 10, avertical shift register 11, n vertical signal lines 12-1 through 12-n, nload registers 13-1 through 13-n, a noise suppression circuit 14, nhorizontal transistors 15-1 through 15-n, and a horizontal shiftregister 16. Each photosensitive cell includes a photodiode 1, atransfer gate 2, a floating diffusion layer section 3, an amplifiertransistor 4, and a reset transistor 5. Of four photosensitive cells onthe first and second rows and columns in FIG. 1, the components includedin the same photosensitive cell are provided with the same suffix (athrough d) for identification. The photosensitive cell has a feature ofincluding three transistors (the transfer gate 2, the amplifiertransistor 4, and the reset transistor 5) and not including an addresstransistor. In practice, values of m and n in the sensor are of theorder of several tens to thousands.

The photosensitive cells arranged in an m×n matrix are formed on asemiconductor substrate. In more detail, the photosensitive cells areformed on a P-type substrate or in a P-well on an N-type substrate. Ineach photosensitive cell, the photodiode 1 performs optical-electricalconversion on received light, and then stores a signal electric charge.The transfer gate 2 is provided between the photodiode 1 and thefloating diffusion layer section 3 to transfer the signal electriccharge stored in the photodiode 1 to the floating diffusion layersection 3. The floating diffusion layer section 3 temporarily stores thesignal electric charge transferred from the photodiode 1. The amplifiertransistor 4 amplifies the signal electric charge stored in the floatingdiffusion layer section 3. The reset transistor 5 resets the signalelectric charge stored in the floating diffusion layer section 3.

On a photosensitive region where these photosensitive cells arearranged, the power supply line 10 and vertical signal lines 12-1through 12-n are routed. Furthermore, signal lines of two types, thatis, m signal lines 17-1 through 17-m and m signal lines 18-1 through18-m, are routed. The power supply line 10 is commonly connected to thedrains of the amplifier transistors 4 of all photosensitive cells. Inthis example, it is assumed that the power supply line 10 is commonlyconnected at one end to the drains of the amplifier transistors 4 andthe drains of the reset transistors 5 included in all photosensitivecells. Also, it is assumed that every photosensitive cell is appliedwith a pulse-type source voltage VddC supplied through a power supplyterminal 20 located at the other end of the power supply line 10. Notethat, although every photosensitive cell is connected to the singlepower supply line 10 in FIG. 1, two or more power supply lines can beused for supplying power to every photosensitive cell.

The vertical signal lines 12-1 through 12-n are respectively providedfor the columns of the photosensitive cells. The vertical signal lines12-1 through 12-n each connect a corresponding one of the amplifiertransistors 4 included in the photosensitive cells arranged in the samecolumn, a corresponding one of the load transistors 13-1 through 13-n,and the noise suppression circuit 14 together. The signal lines 17-1through 17-m and the signal lines 18-1 through 18-m are output signallines of the vertical shift register 11, and are respectively providedfor the rows of the photosensitive cells. The signal lines 17-1 through17-m each connect the gates of the transfer gates 2 included in thephotosensitive cells arranged in the same row. The signal lines 18-1through 18-m each connect the gates of the reset transistors 5 includedin the photosensitive cells arranged in the same row.

The vertical shift register 11 operates as a vertical driver circuit ina manner as follows. When the source voltage VddC is at a high level,the vertical shift register 11 simultaneously drives the transfer gates2 included in the photosensitive cells arranged in the same row. Alsowhen the source voltage VddC is at a high level, the vertical shiftregister 11 simultaneously drives the reset transistors 5 included inthe photosensitive cells arranged in the same row. However, thisoperation of driving the reset transistors 5 is performed at a timedifferent from the time of the above-stated operation of driving thetransfer gates 2. The load transistors 13-1 through 13-n are connectedto the vertical signal lines 12-1 through 12-n, respectively, and arealigned in the row direction. The noise suppression circuit 14 isconnected to the vertical signal lines 12-1 through 12-n, and is fedwith signals output from the amplifier transistors 4 to eliminate noisecomponents included in the fed signals. The horizontal transistors 15-1through 15-n are arranged in the row direction. The horizontaltransistors 15-1 through 15-n are each supplied with the correspondingone of n signals output from the noise suppression circuit 14. Thehorizontal shift register 16 operates as a horizontal driver circuit.That is, the horizontal shift register 16 sequentially and selectivelycauses the horizontal transistors 15-1 through 15-n to operate. Withthis, n signals output from the noise suppression circuit 14 aresequentially output from an output terminal 21.

FIGS. 2A and 2B are illustrations for describing in detail the no sesuppression circuit 14. As illustrated in FIG. 2A, the noise suppressioncircuit 14 includes n sample/hold transistors 31-1 through 31-n, n clampcapacitors 32-1 through 32-n, n clamp transistors 33-1 through 33-n, andn sample/hold capacitors 34-1 through 34-n. The noise suppressioncircuit 14 operates in a manner similar to that of the noise suppressioncircuit illustrated in FIG. 9, although the sample/hold transistors 31-1through 31-n are positioned differently from their counterparts in FIG.9. The gate of each of the sample/hold transistors 31-1 through 31-n isapplied with a sample/hold control signal input from a control terminal22. Similarly, the gate of each of the clamp transistors 33-1 through33-n is applied with a clamp control signal input from a controlterminal 23. These two control signals are changed as illustrated inFIG. 2B. A period during which these two control signals are both at ahigh level is a noise output period. A period during which thesample/hold control signal is at a high level and the clamp controlsignal is at a low level is a signal output period.

With reference to a timing chart illustrated in FIG. 3 as required, ascheme of driving the sensor illustrated in FIG. 1 is described below.In order to drive this sensor, the scheme to be performed includes: astep of pulse driving the power supply line 10 for each horizontalperiod; a step of reading, by the vertical shift register 11, signalsfrom one row of the m×n photodiodes 1; and a step of sequentiallyoutputting, by the horizontal shift register 16, the signals read fromthe one row.

As illustrated in FIG. 3, in the initial state, the source voltage VddCis at a low level. That is, in the initial state, the power supply line10 has not yet been driven. In order to extract signals from thephotosensitive cells on the first row, the source voltage VddC is firstcontrolled to be at a high level. This causes the drains of the transfergate 2 and the reset transistor 5 of every photosensitive cell to be ata high level. Next, while the power supply line 10 is being driven, thevertical shift register 11 causes the signal line 18-1 to be at a highlevel for a predetermined time period. This causes the gate potentialsof the reset transistors 5, including the reset transistors 5 a and 5 b,of the photosensitive cells on the first row to be at a high level,thereby causing these reset transistors 5 to be in an ON state. At thistime, the amplifier transistors 4, including the amplifier transistors 4a and 4 b, of the photosensitive cells on the first row are also in anoperating state. At the same time, noise outputs appear on the verticalsignal lines 12-1 through 12-n. These noise outputs occur when thesignal electric charges stored in the floating diffusion layer sections3, including the floating diffusion layer sections 3 a and 3 b, of thephotosensitive cells on the first row are reset.

Next, while the power supply line 10 is being driven, the vertical shiftregister 11 causes the signal line 17-1 to be at a high level for apredetermined time period. This causes the gate potentials of thetransfer gates 2, including the transfer gates 2 a and 2 b, of thephotosensitive cells on the first row to be at a high level, therebycausing these transfer gates 2 to be in an ON state. At this time, thesignal electric charges stored in the photodiodes 1, including thephotodiodes 1 a and 1 b, of the photosensitive cells on the first roware read to the floating diffusion layer sections 3. Signal outputswhich correspond to the read signal electric charges then appear on thevertical signal lines 12-1 through 12-n.

In this way, on each of the vertical signal lines 12-1 through 12-n, anoise voltage and then a sum of the signal voltage and the noise voltageappear. The noise suppression circuit 14 then operates similarly to aconventional noise suppression circuit, suppressing noise of the signalsoutput to the vertical signal lines 12-1 through 12-n. The noisesuppression circuit 14 then outputs n signals to the respectivehorizontal transistors 15-1 through 15-n.

After the operation of the noise suppression circuit 14, the sourcevoltage VddC is changed to be at a low level. Next, while the powersupply line 10 is not being driven, the vertical shift register 11causes the signal line 18-1 to be at a high level for a predeterminedtime period. This causes the signal electric charges stored in thefloating diffusion layer sections 3, including the floating diffusionlayer sections 3 a and 3 b, of the photosensitive cells on the first rowto be reset. Also, the amplifier transistors 4, including the amplifiertransistors 4 a and 4 b, of the photosensitive cells on the first roware set to be a non-operating state until they are selected next time.

The horizontal shift register 16 then produces n output signals whichare coupled to the gates of the horizontal transistors 15-1 through15-n. The horizontal shift register 16 selectively causes these n outputsignals to be at a high level, thereby sequentially and selectivelycontrolling the horizontal transistors 15-1 through 15-n to be in an ONstate. With this, signals corresponding to the signal electric chargesstored in the photodiodes 1, including the photodiodes 1 a and 1 b, onthe first row are sequentially output from the output terminal 21.

Next, in order to extract signals from the photosensitive cells on thesecond row, an operation similar to that performed on those on the firstrow is performed on those on the second row. With this, signalscorresponding to the signal electric charges stored in the photodiodes1, including the photodiodes 1 c and 1 d, on the second row aresequentially output from the output terminal 21. Then, the same goes forthe photosensitive cells on the third through m-th rows. Note that thedefinitions of a horizontal blanking period, a horizontal effectiveperiod, one horizontal period, and one frame period illustrated in FIG.3 are the same as those in the conventional sensor. Also, as with theconventional sensor, the sensitivity of each photodiode 1 of theexemplary sensor is constant.

The layout pattern of the photosensitive cells included in the exemplarysensor of FIG. 1 can be as illustrated in FIG. 4, for example, if noparticular consideration is given to the layout. In FIG. 4, a portionsurrounded by a dotted line corresponds to one photosensitive cell.Also, in each photosensitive cell, an area surrounded by a thin solidline represents an active region 200, hatched areas representpolysilicon wires 211 through 213, black thick lines represent metalwires 221 through 224, and squares with diagonal lines represent contactholes. Note that the above notation is also applied to other layoutdiagrams described further below.

The active region 200 is surrounded by a device isolation region (notshown), and has formed therein devices serving as circuits and theirelectrodes, such as photodiodes, and the gate, source, and drain of eachtransistor. In the layout pattern illustrated in FIG. 4, eachphotosensitive cell includes one active region 200.

In portions where the active region 200 and the polysilicon wires 211through 213 overlap with each other, transistors are formed. In FIG. 4,in each photosensitive cell, the active region 200 and the polysiliconwires 211 through 213 overlap with each other at three portions.Specifically, in a portion where the active region 200 and thepolysilicon wire 211 overlap with each other, a transfer gate 202 (thetransfer gate 2 in FIG. 1) is formed. In a portion where the activeregion 200 and the polysilicon wire 212 overlap with each other, a resettransistor 205 (the reset transistor 5 in FIG. 1) is formed. In aportion where the active region 200 and the polysilicon wire 213 overlapwith each other, an amplifier transistor 204 (the amplifier transistor 4in FIG. 1) is formed.

Of the active region 200, an area defined by the transfer gate 202 andthe reset transistor 205 corresponds to a floating diffusion layersection 203 (the floating diffusion layer section 3 in FIG. 1). Also, ofthe active region 200, an area located on the opposite side of thetransfer gate 202 from the floating diffusion layer section 203corresponds to a photodiode 201 (the photodiode 1 in FIG. 1).

With the three transistors and the floating diffusion layer section 203formed in the above-described manner being electrically connected toeach other with a predetermined scheme, the photosensitive cellsillustrated in FIG. 1 can be achieved. In the layout pattern illustratedin FIG. 4, connection is achieved by metal wires. Specifically, metalwires of five types are used for each photosensitive cell, and those offour types, that is, the metal wires 221 through 224, are shown in FIG.4. The metal wire 221 connects the floating diffusion layer section 203and the gate of the amplifier transistor 204 both included in the samephotosensitive cell. The metal wire 222 connects the polysilicon wires211 included in the photosensitive cells adjacent to each other in therow direction. These polysilicon wires 211 and the metal wire 222 formthe signal line 17 illustrated in FIG. 1. The metal wire 223 connectsthe polysilicon wires 212 included in the photosensitive cells adjacentto each other in the row direction. These polysilicon wires 212 and themetal wire 223 form the signal line 18 illustrated in FIG. 1. The metalwire 224 connects the sources of the amplifier transistors 204 includedin the photosensitive cells arranged on the same column. The metal wire224 forms the vertical signal line 12 illustrated in FIG. 1. Note thatthe power supply line 10 illustrated in FIG. 1 is not shown in FIG.

In semiconductor manufacturing, the active region 200, the polysiliconwires 211 through 213, and the metal wires 221 through 224 are formed indifferent processes. In order to electrically connect the region andwires of these three types, contact holes are required for connectinglayers. In the layout pattern illustrated in FIG. 4, each photosensitivecell is provided with eight contact holes.

As described above, in the layout pattern illustrated in FIG. 4, thepolysilicon wires 211 and the metal wire 222 forms the signal line 17illustrated in FIG. 1. This is because extending the polysilicon wire211 to a portion of the metal wire 222 causes another overlapping of theactive region 200 and the polysilicon wire, thereby forming an unwantedtransistor in this overlapping portion. However, when the polysiliconwires 211 and the metal wire 222 are used, two contact holes 231 and 232are required in each photosensitive cell for connecting these wirestogether. Accordingly, the metal wire 221 has to be routed so as to goaround the contact hole 232. Furthermore, a contact hole 233 has to beprovided on the metal wire 223 side (on the right side in FIG. 4) of themetal wire 221, thereby disadvantageously increasing the size of thephotosensitive cells in the horizontal direction. As such, in the layoutpattern illustrated in FIG. 4, with the signal line 17 being formed bytwo types of wires, the size of the photosensitive cells in thehorizontal direction is disadvantageously increased.

With particular consideration being given in order to get around theabove disadvantage, the layout pattern of the photosensitive cellsincluded in the exemplary sensor of FIG. 1 can be as illustrated in FIG.5, for example. In this layout pattern, the signal line 17 is achievedonly by the polysilicon wire 211. Therefore, the layout pattern of FIG.5 does not require the contact holes 231 and 232 included in the layoutpattern of FIG. 4, thereby reducing the size of the photosensitive cellsin the horizontal direction, compared with the layout pattern of FIG. 4.

In the layout pattern illustrated in FIG. 5, however, the polysiliconwires 212 and the metal wire 223 form the signal line 18 illustrated inFIG. 1. By forming the signal line 18 only with the polysilicon wire212, the size of the photosensitive cells can be further reduced.

The following descriptions of a sensor according to one embodiment ofthe present invention focuses on a new circuit configuration ofphotosensitive cells for achieving a suitable layout, and the layoutresults of the photosensitive cells having this circuit configuration.

FIG. 6 is a circuit diagram of the sensor according to the embodiment ofthe present invention. The sensor according to the embodiment isdifferent from the exemplary sensor (FIG. 1) only in circuitconfiguration of the photosensitive cells. Therefore, FIG. 6 mainlyillustrates photosensitive cells (each surrounded by a dotted line)arranged in a 2×2 matrix, and does not illustrate the same circuits asthose in FIG. 1 (the vertical shift register 11, the load transistor13-1 through 13-n, the noise suppression circuit 14, the horizontaltransistors 15-1 through 15-n, and the horizontal shift register 16).

In the sensor according to the embodiment, as with the exemplary sensor,each photosensitive cell includes a photodiode 1, a transfer gate 2, afloating diffusion layer section 3, an amplifier transistor 4, and areset transistor 5. These five components function in a manner similarto that of the exemplary sensor.

The photosensitive cell included in the sensor according to theembodiment is different from that included in the exemplary sensor inthe following point. That is, in the exemplary sensor, as describedabove, the floating diffusion layer section 3 included in onephotosensitive cell is connected to the gate of the amplifier transistor4 included in the same photosensitive cell. By contrast, in the sensoraccording to the embodiment, as illustrated in FIG. 6, the floatingdiffusion layer section 3 included in a given photosensitive cell isconnected to the gate of the amplifier transistor 4 included in anotherphotosensitive cell adjacent to the given photosensitive cell in the rowdirection (in FIG. 6, a photosensitive cell directly below the givenphotosensitive cell). For example, in FIG. 6, the floating diffusionlayer section 3 a included in the photosensitive cell shown on the upperleft is connected to the gate of the amplifier transistor 4 c includedin the photosensitive cell located on the lower left. Similarly, thefloating diffusion layer section 3 c included in the photosensitive cellshown on the lower left is connected to the gate of the amplifiertransistor 4 e included in the photosensitive cell (only a portionthereof is shown in FIG. 6) located directly below the lower-leftphotosensitive cell.

As such, with the floating diffusion layer section 3 being connected tothe gate of the amplifier transistor 4 included in anotherphotosensitive cell, a signal output corresponding to the signalelectric charge stored in the photodiode 1 appears on the verticalsignal line 12 by means of the operation of the amplifier transistor 4included in the other photosensitive cell. Even with this circuitconfiguration, the entire operation of the sensor is the same as that ofthe exemplary sensor, as long as these two photosensitive cells arearranged on the same column. Therefore, by applying the scheme ofdriving the exemplary sensor (refer to FIG. 3) to the sensor accordingto the embodiment, an electrical signal corresponding to an opticalsignal supplied to the sensor can be correctly read even from the sensoraccording to the embodiment.

The layout pattern of the photosensitive cells of FIG. 6 can be asillustrated in FIG. 7. In this layout pattern, as with the layoutpatterns in FIGS. 4 and 5, one active region 100 is included in eachphotosensitive cell. In each photosensitive cell, the active region 100and polysilicon wires 111 through 113 overlap with each other at threeportions, where three transistors are formed in each photosensitivecell. Specifically, in the portion where the active region 100 and thepolysilicon wire 111 overlap with each other, a transfer gate 102 (thetransfer gate 2 in FIG. 6) is formed. In the portion where the activeregion 100 and the polysilicon wire 112 overlap with each other, a resettransistor 105 (the reset transistor 5 in FIG. 6) is formed. In theportion where the active region 100 and the polysilicon wire 113 overlapwith each other, an amplifier transistor 104 (the amplifier transistor 4in FIG. 6) is formed.

Of the active region 100, an area defined by the transfer gate 102 andthe reset transistor 105 corresponds to a floating diffusion layersection 103 (the floating diffusion layer section 3 in FIG. 6). Also, ofthe active region 100, an area located on the opposite side of thetransfer gate 102 from the floating diffusion layer section 103corresponds to a photodiode 101 (the photodiode 1 in FIG. 6).

In the layout pattern illustrated in FIG. 7, metal wires 121 through 123of three types are used in each photosensitive cell. The metal wire 121connects the floating diffusion layer section 103 included in a givenphotosensitive cells and the gate of the amplifier transistor 104included in another photosensitive cell adjacent to the givenphotosensitive cell in the column direction. The metal wire 122 connectsthe sources of the amplifier transistors 104 included in thephotosensitive cells arranged on the same column, thereby forming thevertical signal line 12 illustrated in FIG. 1. The metal wire 123connects common drains shared by the amplifier transistors 104 and thereset transistors 105 included in the photosensitive cells arranged onthe same column, thereby achieving a part of the power supply line 10 (aportion extending in the column direction) illustrated in FIG. 1. Notethat the layout pattern of FIG. 7 does not require metal wirescorresponding to the metal wires 222 and 223 included in the layoutpattern of FIG. 4.

In semiconductor manufacturing, the active region 100, the polysiliconwires 111 through 113, and the metal wires 121 through 123 are formed indifferent processes. In order to electrically connect the region andwires of these three types, four contact holes 131 through 134 areprovided for each photosensitive cell in the layout pattern illustratedin FIG. 7.

Compared with the layout patterns of the photosensitive cells includedin the exemplary sensor (FIGS. 4 and 5), the layout pattern of thephotosensitive cells included in the sensor according to the embodiment(FIG. 7) has the following features. In the layout pattern of theexemplary sensor, the photosensitive region is laid out on a rectangulararea. By contrast, in the layout pattern of the sensor according to theembodiment, the photosensitive region is laid out on an area joining tworectangles together (a portion surrounded by a dotted line) inaccordance with the shape of the active region 100. In more detail, itis assumed herein that three photosensitive cells A, B, and C areprovided, and the photosensitive cell B is adjacent to thephotosensitive cell A in the row direction, and the photosensitive cellC is adjacent to the photosensitive cell A in the column direction.Here, consider a case where each photosensitive cell is divided into afirst region and a second region, the first region including thephotodiode 101, the transfer gate 102, and a part of the floatingdiffusion layer section 103 (in FIG. 7, a region on the left side of themetal wire 122) and the second region including the remaining floatingdiffusion layer section 103, the amplifier transistor 104, and the resettransistor 105 (in FIG. 7, a region on the right side of the metal wire122). In this case, broadly speaking, the second region of thephotosensitive cell C is laid out between the first region of thephotosensitive cell A and the first region of the photosensitive cell B.This makes it easy to connect the floating diffusion layer section 103included in a given photosensitive cell and the gate of the amplifiertransistor 104 included in another photosensitive cell adjacent to thegiven photosensitive cell in the column direction.

Also, in the layout pattern of the exemplary sensor, at least either oneof the signal lines 17 and 18 illustrated in FIG. 1 is formed by both ofthe polysilicon wire and the metal wire. By contrast, in the layoutpattern of the sensor according to the embodiment, the signal line 17 isformed only by the polysilicon wire 111, and the signal line 18 isformed only by the polysilicon wire 112. In other words, the signallines 17 and 18 are made of the same material. Furthermore, the floatingdiffusion layer section 103 is located between the polysilicon wire 111connected to the transfer gate 102 included in the same photosensitivecell and the polysilicon wire 112 connected to the reset transistor 105included in the same photosensitive cell. As such, the signal line 17and the gate electrode of the transfer gate 102, and the signal line 18and the gate electrode of the reset transistor are respectively made ofthe same material. Therefore, in order to route these signal lines, nocontact holes are required. This reduces the size of the laid-outphotosensitive cells, thereby reducing the circuit size of the entiresensor.

Furthermore, in the layout pattern of the sensor according to theembodiment, as described above, the metal wires 121 through 123 of threetypes are used, and the four contact holes 131 through 134 are provided.The contact hole 131 is provided on the floating diffusion layer section103 so as to connect the floating diffusion layer section 103 and thegate of the amplifier transistor 104 together. The contact hole 132 isprovided on a common drain shared by the amplifier transistor 104 andthe reset transistor 105 so as to connect the common drain to the metalwire 123 (the power supply line 10 in FIG. 1). The contact hole 133 isprovided on the source of the amplifier transistor 104 so as to connectthe source to the metal wire 122 (the vertical signal line 12 in FIG.1). The contact hole 134 is provided on the same wiring region as thatof the gate of the amplifier transistor 104 so as to connect thefloating diffusion layer section 103 and the gate of the amplifiertransistor 104 together. In the layout pattern of the sensor accordingto the embodiment, these four contact holes 131 through 134 are alignedapproximately in one straight line. This can reduce an area required forlayout of these four contact holes. Therefore, it is possible to reducethe size of the laid-out photosensitive cells, thereby reducing thecircuit size of the entire sensor.

Still further, the metal wire 121, which connects the floating diffusionlayer section 103 and the gate of the amplifier transistor 104 together,the metal wire 122 (the vertical signal line 12 in FIG. 1), and themetal wire 123 (the power supply line 10 in FIG. 1) are all made ofmetal. Thus, these wires of three types can be formed by using the samemetal wire layer. Therefore, no contact holes are required for routingthese signal lines. Therefore, it is possible to reduce the size of thelaid-out photosensitive cells, thereby reducing the circuit size of theentire sensor.

Still further, the metal wires 122 and 123 are routed in parallel on thephotosensitive regions having arranged thereon the photosensitive cells,with the metal wire 121 being located between the metal wires 122 and123. In more detail, the metal wire 121 connecting the floatingdiffusion layer section 103 and the gate of the amplifier transistor 104is located between the metal wire 122 connected to the amplifiertransistor 104 included in the photosensitive cell that includes theabove floating diffusion layer section 103, and the metal wire 123connected to the drain of the above amplifier transistor 104. This makesthe layout pattern of the photosensitive cells simple and regular.Therefore, it is possible to reduce the size of the laid-outphotosensitive cells, thereby reducing the circuit size of the entiresensor.

In this way, the photosensitive cells included in the sensor illustratedin FIG. 6 can be laid out regularly with the least amount of spacewasted. Furthermore, unlike the layout patterns illustrated in FIGS. 4and 5, the layout pattern illustrated in FIG. 7 does not have any metalwires routed on the photodiode. Therefore, according to the embodiment,by adopting a circuit configuration of the sensor for achieving asuitable layout of the photosensitive cell each formed by threetransistors, it is possible to reduce the size of the photosensitivecells and, in turn, reduce the size of the entire sensor. This offerseffects of improving yields of the sensor and reducing the cost of thesensor.

FIG. 8 is an illustration showing another layout pattern of thephotosensitive cells illustrated in FIG. 6. The layout pattern of FIG. 8is different from that of FIG. 7 in the following point. That is, in thelayout pattern illustrated in FIG. 7, the four contact holes 131 through134 included in each photosensitive cell are aligned approximately inone straight line. By contrast, in the layout pattern illustrated inFIG. 8, of the four contact holes 131 through 134 included in eachphotosensitive cell, three contact holes 131 through 133 are alignedapproximately in one straight line, but the contact hole 134 is out ofthis straight line. Depending on the design rules applied for layout ofthe sensor, the layout illustrated in FIG. 8 can be employed.

In the sensor according to the embodiment, all of the transistorsincluded in each photosensitive cell are preferably N-channel MOStransistors because of the following reason. In recent years, with alarge majority of logic circuits being manufactured by using CMOS,MOS-type solid-state imaging devices are also often manufactured byusing CMOS. Since a CMOS process for logic circuits includes a largenumber of complex processes, it is extremely difficult to change even apart of these processes only for the purpose of manufacturing thesensor. Therefore, in order to manufacture the sensor, an additionalprocess unique to the sensor has to be added to the existingmanufacturing processes. In this case, boron, which is a p-typeimpurity, is light in mass and is freely movable, and therefore isdifficult to be made small inside the semiconductor. In consideration ofthis, it is advantageous to use only NMOS in the process unique tomanufacturing of the sensor.

As described in the foregoing, according to the sensor of the embodimentof the present invention, by adopting a new circuit configuration forachieving a suitable layout, it is possible to reduce the size of thephotosensitive cells, thereby reducing the size of the entire sensor.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. An imaging device formed in a semiconductorsubstrate, comprising: a plurality of photodiodes arranged in rows andcolumns, each photodiode for a given row of photodiodes formed in acorresponding active region of the semiconductor substrate; a transfergate disposed over the corresponding active region so as to define anassociated photodiode on one side of the transfer gate and an associatedfloating diffusion element on another side of the transfer gate; anamplifier transistor; a reset transistor; a first wire electricallyconnecting the associated floating diffusion element to a gate electrodeof the amplifier transistor, a first contact hole connected to a secondwire disposed over the semiconductor substrate and being for receivingan output of a source of the amplifier transistor; and a second contacthole connecting a drain of the reset transistor to a third wire disposedover the first semiconductor substrate, wherein the amplifier transistoris formed in another active region which is separated from thecorresponding active region where the associated floating diffusionelement connected to the gate electrode of the amplifier transistor isformed, the first wire is disposed in a column direction between theassociated floating diffusion element and the gate electrode of theassociated amplifier transistor, and the first contact hole and thesecond contact hole are aligned substantially in the column direction.2. The imaging device according to claim 1, wherein the first wireincludes a straight portion disposed in the column direction.
 3. Theimaging device according to claim 1, wherein the first wire includes aportion disposed in a direction traversing column direction.
 4. Theimaging device according to claim 1, wherein the first wire includes astraight portion disposed in a first column direction and anotherportion disposed in a direction traversing column direction, and thestraight portion is connected to the associated floating diffusionelement.
 5. The imaging device according to claim 1, wherein transfergates corresponding to adjacent photodiodes are formed by a continuousconducting material layer.
 6. The imaging device according to claim 1,further comprising: a third contact hole connecting the associateddiffusion element to the first wire, wherein the first, second, andthird contact holes are aligned substantially in a straight line.
 7. Theimaging device according to claim 6, further comprising: a fourthcontact hole connecting the gate electrode of the amplifier transistorto the first wire.
 8. The imaging device according to claim 7, whereinthe first, second, third, and fourth contact holes are alignedsubstantially in a straight line.
 9. The imaging device according toclaim 7, wherein the fourth contact hole is out of alignment withrespect to the first, second, and third contact holes.
 10. The imagingdevice according to claim 8, wherein the amplifier transistor isarranged between adjacent photodiodes in a row direction.
 11. Theimaging device according to claim 8, wherein the transfer gate and thereset transistor share a common drain.
 12. The imaging device accordingto claim 8, wherein the first wire is disposed in approximately astraight line between the associated floating diffusion element and thegate electrode of the amplifier transistor.
 13. The imaging deviceaccording to claim 8, further comprising: a fifth contact hole connectedto a source of the reset transistor, wherein the first, second, andfifth contact holes are aligned substantially in a straight line. 14.The imaging device according to claim 8, further comprising: a fifthcontact hole connected to a source of the reset transistor; and a sixthcontact hole connected, to supply an input to a drain of the amplifiertransistor, to a third wire disposed over the first semiconductorsubstrate; and wherein the first, fifth, and sixth contact holes arealigned substantially in a straight line.
 15. An imaging device formedin a semiconductor substrate, comprising: a plurality of photodiodesarranged in rows and columns, each photodiode for a given row ofphotodiodes formed in a corresponding active region of the semiconductorsubstrate; a transfer gate disposed over the corresponding active regionso as to define an associated photodiode on one side of the transfergate and an associated floating diffusion element on another side of thetransfer gate; an amplifier transistor; a reset transistor; a first wireelectrically connecting the associated floating diffusion element to agate electrode of the amplifier transistor, a third contact holeconnecting the associated diffusion element to the first wire; and asixth contact hole connected, to supply an input to a drain of theamplifier transistor, to a third wire disposed over the firstsemiconductor substrate; wherein the amplifier transistor is formed inanother active region which is separated from the corresponding activeregion where the associated floating diffusion element connected to thegate electrode of the amplifier transistor is formed, the first wire isdisposed in a column direction between the associated floating diffusionelement and the gate electrode of the associated amplifier transistor,and the third contact hole and the sixth contact hole are alignedsubstantially in the column direction.
 16. The imaging device accordingto claim 15, wherein the first wire includes a straight portion disposedin the column direction.
 17. The imaging device according to claim 15,wherein the first wire includes a portion disposed in a directiontraversing column direction.
 18. The imaging device according to claim15, wherein the first wire includes a straight portion disposed in afirst column direction and another portion disposed in a directiontraversing column direction, and the straight portion is connected tothe associated floating diffusion element.
 19. The imaging deviceaccording to claim 15, wherein transfer gates corresponding to adjacentphotodiodes are formed by a continuous conducting material layer. 20.The imaging device according to claim 15, further comprising: a firstcontact hole connected to a second wire disposed over the semiconductorsubstrate and being for receiving an output of a source of the amplifiertransistor; and a second contact hole connecting a drain of the resettransistor to a third wire disposed over the first semiconductorsubstrate; wherein the first, second, and third contact holes arealigned substantially in a straight line.
 21. The imaging deviceaccording to claim 20, further comprising: a fourth contact holeconnecting the gate electrode of the amplifier transistor to the firstwire.
 22. The imaging device according to claim 20, wherein the first,second, third, and fourth contact holes are aligned substantially in astraight line.
 23. The imaging device according to claim 21, wherein thefourth contact hole is out of alignment with respect to the first,second, and third contact holes.
 24. The imaging device according toclaim 15, wherein the amplifier transistor is arranged between adjacentphotodiodes in a row direction.
 25. The imaging device according toclaim 15, wherein the transfer gate and the reset transistor share acommon drain.
 26. The imaging device according to claim 15, wherein thefirst wire is disposed in approximately a straight line between theassociated floating diffusion element and the gate electrode of theamplifier transistor.
 27. The imaging device according to claim 15,further comprising: a first contact hole connected to a second wiredisposed over the semiconductor substrate and being for receiving anoutput of a source of the amplifier transistor; a second contact holeconnecting a drain of the reset transistor to a third wire disposed overthe first semiconductor substrate; and a fifth contact hole connected toa source of the reset transistor, wherein the first, second, and fifthcontact holes are aligned substantially in a straight line.
 28. Theimaging device according to claim 15, further comprising: a firstcontact hole connected, to receive an output of a source of theamplifier transistor, to a second wire disposed over the semiconductorsubstrate; and a fifth contact hole connected to a source of the resettransistor; wherein the first, fifth, and sixth contact holes arealigned substantially in a straight line.